Assessment of urban flooding in Yen Hoa - Hoa Bang area, Cau Giay, Hanoi

56 Vietnam Journal of Hydrometeorology, ISSN 2525-2208, 2019 (03): 56-61 Tran Kim Chau1, Tran Thi Quynh1, Nguyen Van Minh2, Nguyen Thanh Thuy1 ABSTRACT Hoa Bang street is known as one of flooding hot spots in Hanoi with 30 inundated times dur- ing last 4 years. In 2019, a new conduit has been set up to drain the water from Hoa Bang street to To Lich River. However, the flooding situation has not been significant improved. Therefore, it is necessary to evaluate the capacity of the drainage systems as well as effectiveness of flooding mitigation measures. In this research, the numerical model MIKE URBAN is used to simulate the rainfall-runoff, routing and sur- charge processes in Yen Hoa - Hoa Bang areas. Water depth and flood durations are indicators to assess the performance of the proposed solu- tions. The result of this research indicates that the renovation of the drainage system, i.e. en- largement in conduits’ diameter, can reduce in- undation time significantly. Keywords: MIKE FLOOD, MIKE URBAN, urban flooding, drainage system. 1. Introduction In recent years, in big cities of Vietnam as well as in Hanoi, the population has been grow- ing rapidly and the speed of urbanization have resulted in natural land’s contraction and the spread of concretized land. The fact that many rivers and lakes have been filled, canals have been encroached, and high-rise buildings have been built closely to replace vacant land reduces the area of natural drainage as well as the per- meability and the time of overland flow on the surface. It can be seen that, the urban drainage system of Hanoi is old and not designed to keep up with urban planning of the City. At the same time, the projects to renovate the drainage sys- tem in the inner-city area are still slow due to many reasons. Combined with heavy rains caused by climate change, Hanoi has continu- ously faced with large-scale floods in recent years, which greatly affected socio-economic ac- tivities, especially in the inner-city area. Histor- ical rains occurred in late October and early November 2008 with a total rainfall common from 350 to 550 mm, causing serious inundation throughout Hanoi, many flooded spots appeared. length of 100-300 meters, depth of less than 1m, causing economic losses of up to 3,000 billion. Hoa Bang street is known as one of flooding hot spots in Hanoi. There are 9, 7, 3 11 flooding events with total flooding time of 1062, 1669, 884, 2188 minutes corresponding to year 2016, 2017, 2018 and 2019. The water depth varies from 0.1 to 0.4 m. In recent decades, mathematical models have been increasingly applied to urban flood simula- tion problems. These include 1D sewer model approach, e.g. SWMM (Rossman, 2010), Hy- droPlaner (Fareed, 2013), or coupled with 1D sewer model with 2D surface flow models, e.g. Research Paper ASSESSMENT OF URBAN FLOODING IN YEN HOA - HOA BANG AREA, CAU GIAY, HANOI ARTICLE HISTORY Received: November 12, 2019 Accepted: December 20, 2019 Publish on: December 25, 2019 TRAN KIM CHAU Corresponding author: kimchau_hwru@tlu.edu.vn 1Thuyloi University 2Power Engineering Consulting Joint Stock Company 1 un- d Accepted: November 12, 2019 P B DOI:10.36335/VNJHM.2019(3).56-61 57 SOBEK 1D/2D (Deltares Delft Hydraulics, 2019), InfoWorks ICM (Innovyze 2019). Ac- cording Leandro (2009), overflow is better mod- eled by 2D models, whereas 1D models provide a good approximation flow in pipe. Pham et al. (2015) applied MIKE URBAN to simulate the inundation in 8 inner districts of Hanoi. In their research, only main drainage routs were con- ducted in the networks. This means the drainage capacity of the tributaries were not considered. In this research, the coupled with 1D sewer model with 2D surface flow model has been per- formed for Yen Hoa - Hoa Bang area with a de- tailed drainage system (Fig. 1). Yen Hoa - Hoa Bang area is located in terri- tory of Cau Giay district. The case study covers an area of 40.47 ha. This is one of the lowest areas in Hanoi but the topography is complex with the altitude from 4.4 m to 7.8 m. The heavy rains appear frequently in the summer with maximum hourly precipitation varying from 31.9 mm to 114.9 mm. The main drainage routs are located along Hoa Bang, Yen Hoa, alley 381 of Nguyen Khang street, the Nguyen Khang street from Cot bridge to Yen Hoa bridge. The drainage systems discharge flow to To Lich river at 2 outlets. 2. Materials and Methods The research was implemented in 3 main steps (Fig. 2). 2.1 Data collection The collected data includes meteorological data, inundated depth and drainage system in- formation. 10 minute rainfall data of heavy rains since June to September of 2019 at Cau Giay rain gauge were assembled. During this period, the study area were most suffered from the 2 heavy rainfall events at the end of April and early August 2019. Therefore, these 2 rain- fall events were selected for calibration and validation correspondingly. Besides the mete- orological data, the information on the drainage system updated to August 2019 were collected. This information includes locations of water collection stations, sewer diameter, manhole diameter, surface elevation, eleva- tion of manhole bottom, slope slope ... More- over, the 1/10000 scale topo data of the area has also been collected. In order to conduct validation process, flood marks corresponding to the above flood events were also studied and collected. 2.2 Description of MIKE FLOOD model To conduct the simulations, MIKE FLOOD model was used for calculation. In this model, MIKE URBAN and MIKE 2D FM are linked together. MIKE URBAN model works based on a link of hydrological models and hy- draulic models (DHI 2014c). The model struc- ture can be described as Fig. 3. Tran Kim Chau et al./Vietnam Journal of Hydrometeorology, 2019 (03): 56-61














































































































































































































































       













      
























































































































































Fig. 1. Map of the study area
































































































































































































































       













      





















































































































































Fig. 2. The scheme of research implementation Assessment of urban flooding in Yen Hoa - Hoa Bang area, Cau Giay, Hanoi 58 In this study, the calculated area was divided into 414 sub-basins. In each of these sub-basins, the Time-Area (T-A) method was used to con- vert the precipitation that falls on the sub-basin into the flow which discharges into the culverts (DHI, 2014a, 2014b). The hydraulic module routs flow in the sewer system by solving Saint Venant equations (Eq. 1, Eq. 2). These two equations are applied for free sur- face flow. For the pressurised flow, the fictitious slot is introduced and the continuous equation Eq. 1 can be written as Eq. 3. 2.3 Setup model In the study area, a network of 461 sluices are simulated for the three main streets of Hoa Bang, Yen Hoa and a part of Nguyen Khang street. In addition, some main lane routes are also modeled. Roughness coefficients of sewer system are taken according to the instructions in TCVN 7957: 2008 (VIWASE, 2008). The water in the system is dis- charged to the To Lich river through 2 outlets. The drainage network of the area is shown in Fig. 4. MIKE URBAN is combined with MIKE 2D FM to account for flooding when the drainage capacity of the system does not meet the runoff from in the catchments. The mesh of this 2D model was constructed from detailed grid cells with an area of 1 to 5 m2. MIKE URBAN and MIKE 2D FM are connected at the manholes. The water level at the manholes is calculated at each time and compared with the water level in the 2D grid at that location. If there is a water level difference, the water exchange between the two models will be performed. The data collected for the rain since April 29th, 2019 to April 30th, 2019 was used for calibration and the data since August 1st, 2019 to August 4th, 2019 was used for validation. The simulation re- sults were compared with observed inundated depths to ensure the accuracy of the model. After calibration and validation, the model will be used to simulate the design rains. Besides, a flood mitigation measure was proposed. The cal- culation scenarios are shown in detail as Table 1.
















































      

















































































































































































       



































































































Fig. 3. The structure of MIKE URBAN
















































      




























































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Fig. 5. MIKE FLOOD model for research area 59 Hoang Thi Ngoc Ha et al./Vietnam Journal of Hydrometeorology, 2019 (03): 56-61 3. Results and discussions Tables 2 and 3 shows that the calculation re- sults are quite consistent with the reality at some typical flooding points in both calibration and validation. It was found that the flooding in the study area is still severe with the current drainage sys- tem condition. Especially, the storm event since August 1st, 2019 to August 4th, 2019 affected by typhoon No. 3 with heavy rain on August 3rd and August 4th, 2019 made Hoa Bang area was inundated for a long time. Especially the water depth at alley 90, Hoa Bang street was 0.35m, Lane 35 was 0.23m. Some specific points such as 90 Hoa Bang flooded 0.35m and 35 Hoa Bang flooded 0.29m. there is no flooding in some areas such as Nguyen Khang and Yen Hoa streets because the sewers in these areas have sluices’ diameter from 0.8m to 1m. Additionally, these areas are closed to To Lich River so the amount rainwater quickly drains into the river. The cause of inundation at Hoa Bang street can be caused by two main reasons. First, heavy rains occurred, the drainage capacity of the existing sewer sys- tem could not address. Especially Hoa Bang street, when the drain diameter of the main line is only 0.6m. The second, the concave terrain of this area is also a major factor causing flooding in the area. Figure 8 describes the longitude pro- file along the Hoa Bang route. Looking at the figure, it is noticed that the topography of Hoa Bang Street in the middle creates favorable con- ditions for forming local flooding spots. Over- flow is unable to drain into the river but stays and waits for flow through the public system. Therefore, reducing the depth of flooding in this area is very difficult to overcome. However, it is possible to reduce the flooding time by increas- ing the likelihood of drainage through the sewer system. Based on the analysis, the study proposes mit- igated solution. According to decision 725/QĐ- TTg (Prime Minister 2014), Hanoi urban area must respond to design rain with a frequency of














       
















































































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Table 2. The observation and calculation depth in calibration process (m)